The present invention relates to telephone apparatus fitted with an amplifier and commonly referred as hands-free telephone apparatus.
A hands-free telephone set is one in which a sensitive microphone and a loudspeaker both working in association with high-gain amplifiers are incorporated into the housing of the set thus no longer requiring the user to hold a handset close to his mouth and ear.
This ease of use is nevertheless compensated for by one constraint consisting in the necessity of conversing alternatively in order to avoid feedback effect such as the Larsen effect which would otherwise occur as a result of acoustic, mechanical and electrical coupling between the send and receive channels. Such alternating operation is of course automatic and numerous documents, such as French Patent Application 2 518 854 provide a detailed description of it. In this known art, analog devices are employed which permanently test the respective levels of the signal being sent and received in order to decide which channel should be enabled. This channel is then transmitted normally without any attenuation except for its inherent attenuation while the other channel is subject to appreciable attenuation generally of the order of 40 to 50 dB to give some idea of the order of magnitude. Such discrimination enables a decision to be made as to which channel is sending at the start of conversation and then to possibly change the channel when sending comes to an end, even temporarily. During sending, the other channel cannot in principle be enabled even if an attempt is made to activate it in order to break in while the other user is speaking because it is in any case strongly attenuated.
The analog processing employed in the known devices suffers from the disadvantage that it does not allow fairly rapid detection and comparison of the signals that are present on the send and receive channels. Decision taking which requires a good twenty milliseconds in this case is too slow to prevent the start of the corresponding phrase from being cut off, meaning that there is a loss of information at each start of conversation and each time there is a changeover from one party to the other.
Furthermore, these implementations that use analog components, require a large number of components and a non-negligible amount of printed circuit area, thus handicapping the cost and bulkiness of such telephone sets. The invention sets out to overcome these disadvantages.